The purpose of spacer grids is to space out the nuclear fuel rods in a nuclear reactor assembly. The rods must be retained parallel in a predetermined network, while at the same time ensuring that they are born in accordance with different zones distributed over their length, more particularly in order to prevent them from vibrating as a result of the circulation of a liquid coolant flowing in contact with them.
Numerous constructions are known for spacer grids of the kind specified, inter alia constructions comprising an assembly formed by two families of thin metal sheets, respectively parallel in each family and perpendicular from one family to the next, the sheets being notched at regular intervals to enable them to be fitted into one another, thus bounding pockets of square section through each of which a fuel element of the cluster extends.
Grids of that kind generally have added springs, generally made of a material different from that of the grid. For example, French Pat. No. 77 20 908, filed on July 7, 1977, for "Spacer grid for a cluster of fuel rods in a nuclear reactor assembly" discloses a spacer grid comprising spring tongues having a zig-zag profile which comprises at least two bosses bearing respectively against the rods extending through the two pockets.
The grid is made from zircaloy, the springs being made of inconel.
An attempt has been made to produce grids entirely from zircaloy, thus avoiding the use of materials with a high neutron absorption power, while allowing a simpler construction of the spacer grid, since the assembly of added springs is eliminated.
At first an attempt was made to produce grids having springs cut out directly in the partitions forming the walls of the pockets. However, tests showed that such springs did not enable the fuel rods to be durably retained, due to the relaxation of the zircaloy under irradiation. After some time the clamping force of the springs on the rods is relaxed, and they are no longer retained.
The invention relates more particularly to a spacer grid made entirely from a material having a low neutron absorption power and of very simple construction. Since it is impossible to ensure clamping during the whole life of the rods, the grid merely ensures their spacing. Of course, it remains necessary to ensure the function of retaining the rods. However, this is achieved by another means, which does not form part of the present invention.It is essential to ensure a clearance-free contact between the fuel rods and the grids, since any clearance between the grid and rods would cause the latter to wear, and this must be avoided under all circumstances. The dimensional variations due to the manufacturing tolerances of the grids must therefore be compensated. The invention solves this problem by the presence of opposite pairs of bearing points produced on resiliently deformable lamellae.
It must also be pointed out that U.S. Pat. No. 3,679,547 discloses homogeneous grids in which two superimposed bosses per pocket are produced in a lamella, the bosses each being disposed between two vertical slots and oriented in opposite directions in relation to the lamella, in the direction of two adjoining pockets. In such a construction the torques due to the presence of the rods are transmitted to the lamellae forming the grids; the torques produce in the grids deformations which are prejudicial to their stability and geometry.
It is an object of the invention to obviate these disadvantages.